This invention concerns a lithium soap grease of premium quality with outstanding high temperature and oxidation stability properties.
Lithium greases have been known and widely used for many years. The principal advantages of lithium greases have been high dropping points, good water resistance and ease of dispersion of the soaps in all types of lubricating oil base stocks. While the lithium soaps that are used as thickening agents for these greases can be prepared by reaction of lithium hydroxide or other lithium base with conventional high molecular weight fatty acids, lithium hydroxy stearate and the lithium soaps of related hydroxy fatty acids have been particularly useful because of their great mechanical stability.
Among the various lithium grease formulations which have been disclosed are a grease thickened with the combination of a lithium soap of a C.sub.12 to C.sub.24 hydroxy fatty acid and a lithium soap of a C.sub.2 to C.sub.12 aliphatic dicarboxylic acid as disclosed in U.S. Pat. Nos. 3,681,242 and 3,791,973; a grease with a thickener system comprising a combination of a monolithium salt of boric acid and a lithium soap of C.sub.12 to C.sub.24 hydroxy fatty acid as disclosed in U.S. Pat. No. 3,758,407; a grease composition comprising a lithium soap derived from a fatty acid containing an epoxy group and/or ethylenic unsaturation and a dilithium soap derived from a straight chain dicarboxylic acid as shown in U.S. Pat. No. 3,985,662; and a grease composition with a thickener system comprising a combination of a dilithium salt of a C.sub.4 -C.sub.12 dicarboxylic acid, a lithium soap of C.sub.12 to C.sub.24 hydroxy fatty acid and a lithium salt formed in situ from a hydroxy carboxylic acid as disclosed in U.S. Pat. No. 3,929,651.
A variety of antioxidants have been used in lubricating oils over the years as shown, for example, in U.S. Pat. No. 3,785,977 wherein diarylamines, thiodarylamines and s-triazines are among the antioxidants disclosed; in U.S. Pat. No. 3,671,434 wherein quinone phosphates are disclosed as oxidation and corrosion inhibitor; in U.S. Pat. No. 3,642,632 wherein esters of anthranilic acid are disclosed as oxidation and corrosion inhibitors and in U.S. Pat. No. 2,226,427 wherein selected metal derivatives of glyoximes, particularly nickel, cobalt and iron are disclosed as antioxidants.
Many of the antioxidants which are effective in lubricating oils have been found to be undesirable for use in grease compositions since they tend to detrimentally effect the quality of the grease such as by softening the grease composition or by destroying or breaking down the grease structure. One known antioxidant which is particularly effective for relatively high temperature greases is the alkali metal salts of a hydroxy aromatic acid such as dilithium salicylate as disclosed in U.S. Pat. Nos. 2,951,808 and 3,711,407. However, the use of such antioxidants in grease compositions even at higher concentrations, offers limited life at the higher temperatures of about 350.degree. F. or more and there is generally a need for frequent relubrication.